1. The Field of the Invention
The present invention is related generally to thin film optical coatings for use in producing security articles. More specifically, the present invention is related to the production of diffractive surfaces such as holograms or gratings having color shifting or optically variable backgrounds which can be used as security articles in a variety of applications.
2. The Relevant Technology
Color shifting pigments and colorants have been used in numerous applications, ranging from automobile paints to anti-counterfeiting inks for security documents and currency. Such pigments and colorants exhibit the property of changing color upon variation of the angle of incident light, or as the viewing angle of the observer is shifted. The primary method used to achieve such color shifting colorants is to disperse small flakes, which are typically composed of multiple layers of thin films having particular optical characteristics, throughout a medium such as paint or ink that may then be subsequently applied to the surface of an object.
Diffraction patterns and embossments, and the related field of holographs, have begun to find wide-ranging practical applications due to their aesthetic and utilitarian visual effects. One very desirable decorative effect is the iridescent visual effect created by a diffraction grating. This striking visual effect occurs when ambient light is diffracted into its color components by reflection from the diffraction grating. In general, diffraction gratings are essentially repetitive structures made of lines or grooves in a material to form a peak and trough structure. Desired optical effects within the visible spectrum occur when diffraction gratings have regularly spaced grooves in the range of hundreds to thousands of lines per millimeter on a reflective surface.
Diffraction grating technology has been employed in the formation of two-dimensional holographic patterns which create the illusion of a three-dimensional image to an observer. Three-dimensional holograms have also been developed based on differences in refractive indices in a polymer using crossed laser beams, including one reference beam and one object beam. Such holograms are called volume holograms or 3D holograms. Furthermore, the use of holographic images on various objects to discourage counterfeiting has found widespread application.
There currently exist several applications for surfaces embossed with holographic patterns which range from decorative packaging such as gift wrap, to security documents such as bank notes and credit cards. Two-dimensional holograms typically utilize diffraction patterns which have been formed on a plastic surface. In some cases, a holographic image which has been embossed on such a surface can be visible without further processing; however, it is generally necessary, in order to achieve maximum optical effects, to place a reflective layer, typically a thin metal layer such as aluminum, onto the embossed surface. The reflective layer substantially increases the visibility of the diffraction pattern embossment.
Every type of first order diffraction structure, including conventional holograms and grating images, has a major shortcoming even if encapsulated in a rigid plastic. When diffuse light sources, such as ordinary room lights or an overcast sky, are used to illuminate the holographic image, all diffraction orders expand and overlap so that the diffraction colors are lost and not much of the visual information contained in the hologram is revealed. What is typically seen is only a silver colored reflection from the embossed surface and all such devices look silvery or pastel, at best, under such viewing conditions. Thus, holographic images generally require direct specular illumination in order to be visualized. This means that for best viewing results, the illuminating light must be incident at the same angle as the viewing angle.
Since the use of security holograms has found widespread application, there exists a substantial incentive for counterfeiters to reproduce holograms which are frequently used in credit cards, banknotes, and the like. Thus, a hurdle that security holograms must overcome to be truly secure, is the ease at which such holograms can be counterfeited. One step and two step optical copying, direct mechanical copying and even re-origination have been extensively discussed over the Internet. Various ways to counteract these methods have been explored but none of the countermeasures, taken alone, has been found to be an effective deterrent.
One of the methods used to reproduce holograms is to scan a laser beam across the embossed surface and optically record the reflected beam on a layer of a material such as a photopolymerizable polymer. The original pattern can subsequently be reproduced as a counterfeit. Another method is to remove the protective covering material from the embossed metal surface by ion etching, and then when the embossed metal surface is exposed, a layer of metal such as silver (or any other easily releasable layer) can be deposited. This is followed by deposition of a layer of nickel, which is subsequently released to form a counterfeiting embossing shim.
Due to the level of sophistication of counterfeiting methods, it has become necessary to develop more advanced security measures. One approach, disclosed in U.S. Pat. Nos. 5,624,076 and 5,672,410 to Miekka et al., embossed metal particles or optical stack flakes are used to produce a holographic image pattern.
A further problem with security holograms is that it is difficult for most people to identify and recollect the respective images produced by such holograms for verification purposes. The ability of the average person to authenticate a security hologram conclusively is compromised by the complexity of its features and by confusion with decorative diffractive packaging. Thus, most people tend to confirm the presence of such a security device rather than verifying the actual image. This provides the opportunity for the use of poor counterfeits or the substitution of commercial holograms for the genuine security hologram.
In other efforts to thwart counterfeiters, the hologram industry has resorted to more complex images such as producing multiple images as the security device is rotated. These enhanced images provide the observer with a high level of “flash” or aesthetic appeal. Unfortunately, this added complexity does not confer added security because this complex imagery is hard to communicate and recollection of such imagery is difficult, if not impossible, to remember.
U.S. Pat. No. 5,700,550 issued Dec. 23, 1997 in the name of Uyama et al. discloses a transparent hologram seal having a hologram forming layer embodied by a relief type hologram image, a transparent evaporated layer in the form of a multi-layered ceramic layer having alternatively laminating high-refractive index layers and low-refractive index layers. Uyama discloses 5 layers having a thickness of preferably less than 1 μm as being optimum for color shifting. The notion of using both a hologram relief structure and an optically variable filter provides additional security features to using only a hologram or only a thin film filter. Notwithstanding, an objective in providing a security device having both a hologram and a thin film filter is to allow the user or authenticator to quickly identify these security features so as to validate a document or item which carry them. Furthermore, it would be an object of this invention to provide a synergistic effect by combining a hologram with a thin film optically variable filter. Embodiments of this invention exploit the fact that there is interaction between the diffraction grating and the optically variable structure, which yield desirable and visible effects, not seen in security devices only having a hologram or only having a color shifting filter. By providing a device with high chroma, these effect are enhanced.
Uyama in U.S. Pat. No. 5,700,550 provides a transparent hologram seal that requires an all dielectric stack on the hologram forming layer. Unfortunately, when this device is affixed to a security document having a white background, as for example, a white sheet of paper or a white document, it is found that the transmitted beam will reflect off of the white surface, returning though the device adding to the reflected rays, resulting in a washed out or pale pastel color. The instant invention ameliorates this problem by providing a non-transmissive, substantially opaque device. In contrast the instant device provides reflected colors that are highly saturated, having high chroma irrespective of the color of the document surface. Furthermore, only three color shifting layers are required to achieve this high chroma in contrast to Uyama's five layers. Uyama's patent discloses an attempt at using three transparent dielectric layers however it was reported that a clear color change did not occur. The applicants believe that this lack of clear color change was due to the weak reflectivity of the hologram as a result of using a transparent all dielectric system, with a transparent hologram, so that reflective diffractive peaks were not visible; only those peaks arising from the all dielectric coating were seen, as for example in FIG. 21 of Uyama.
In the instant invention, illustrated in example 5 of the specification the hologram colors are modified such that some of the strong rainbow colors normally emanating from a reflection hologram, as opposed to the transparent hologram of Uyama, are accentuated or suppressed by the Fabry Perot filter. Furthermore, the diffractive pattern is built and replicated into the Fabry Perot reflection filter allowing maximum reflection from both the diffractive and interference devices. This is illustrated in Example 5 of this specification. This color interaction is totally new for a security device and there are currently several Federal governments and holographic companies interested in the device of this invention becoming commercially available.
It would therefore be of substantial advantage to develop improved security products which provide enhanced viewing qualities in various lighting conditions, especially in diffuse lighting, and which are usable in various security applications to make counterfeiting more difficult.